The invention relates to the field of continuous-wave, and synchronously pumped mode locked dye lasers, and, more particularly, to the field of birefringent tuning plates and tuning mechanisms in general for such lasers.
Continuous-wave dye lasers and birefringent plates for tuning the wavelength of lasing activity are known. However, heretofore, there has been no closed form equation for relating the wavelength of minimum loss, i.e., the lasing wavelength, to the tuning angle of the tuning mechanism although, such an equation could be derived from a paper by Lovold, et al. to be discussed in more detail below, at least for the simple cases discussed herein. Also, it has been noticed in prior art laser designs that tuning anomalies consisting of sudden unpredictable jumps in the wavelengths of lasing activity sometimes occur near the ends of the tuning range. To better understand these problems, some background on lasers and the tuning mechanism for same is in order.
A laser is a device that uses the principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the infrared, visible, or ultraviolet region. The term "laser" is an acronym for light amplification by stimulated emission of radiation, or a light amplifier. However, just as an electronic amplifier can be made into an oscillator by feeding appropriately phased output back into the input, so the laser light amplifier can be made into a laser oscillator, which is really a light source. Laser oscillators are so much more common than laser amplifiers that the unmodified word "laser" has come to mean the oscillator, while the modifier "amplifier" is generally used when the oscillator is not intended.
The process of stimulated emission can be described as follows. When atoms, ions, or molecules absorb energy, they can emit light spontaneously (as an incandescent lamp) or they can be stimulated to emit light energy by a light wave. The stimulated emission is the opposite of stimulated absorption, where unexcited matter is stimulated into an excited state by a light wave. If a collection of atoms is prepared (pumped) so that more are initially excited than unexcited, then an incident light wave will stimulate more emission that absorption, and there is net amplification of the incident light beam. This is the way a laser amplifier works.
A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the amplifier. These are called the resonator and are sometimes referred to as defining the resonant cavity within which the lasing material is found. Thus, the essential parts of the laser oscillator are an amplifying medium, a source of pump power, and a resonator. Radiation that is directed straight along the axis of the resonator cavity bounces back and forth between the mirrors and can remain in the resonator long enough to build up a strong oscillation. Radiation may be coupled out of the resonator cavity by making one mirror partially transparent so that part of the amplified light can emerge through it. The output wave, like most of the waves being amplified between mirrors, travels along the axis and is thus very nearly a plane wave.
One way to achieve population inversion is by concentrating light as pump energy onto the amplifying medium. Alternatively, lasers may be used to optically pump other lasers. For example, powerful continuous wave ion lasers can pump liquid dyes to lase, yielding watts of tunable, visible, and near visible coherent radiation. Laser light is coherent in that all light of a given wavelength is in phase by virtue of the stimulated emission nature of generation of the light.
Liquid lasers have structures generally like those of optically pumped solid-state lasers, except that the liquid is generally contained in a transparent cell. Some liquid lasers make use of rare-earth ions and suitable dissolved molecules, while other make use of organic dye solutions. The dyes can lase over a wide range of wavelengths, depending upon the composition and concentration of the dye or solvent. Thus, tunability is obtained throughout the visible, and out to a wavelength of about 1 micron. Fine adjustment of the output wavelength can be provided by using a diffraction grating or other dispersive element in place of one of the laser mirrors or somewhere in the cavity between the mirrors. Where a diffraction grating is substituted for one of the mirrors, the grating acts as a good mirror for only one wavelength, which depends on the angle at which it is set. With further refinements, liquid-dye lasers can be made extremely monochromatic, as well as broadly tunable.
In the prior art, prisms and gratings are currently widely used with dye lasers as tuning elements. These devices are examples of spatially dispersive selectors. The attainment of high resolution with these devices requires large spot sizes at the tuning element and creates serious difficulties in the design of other parts of the laser. Tilted etalons can be used to give narrow bandwidths at particular points in the spectrum, but any one etalon has a very limited tuning range. Electrooptically tuned Lyot filters have also been used to tune laser output wavelength.
Tilted birefringent plates have also been successfully demonstrated for use as highly tunable, narrow-band selection devices. This is taught in Bloom, "Modes of a Laser Resonator Containing Tilted Birefringent Plates", Journal of the Optical Society of America, Vol. 64, No. 4, pp. 447-452 (April 1974). The resonator in such a device contains at least one pair of surfaces oriented at Brewster's angle to the incident light rays, and a birefringent plate that is tilted and whose optic axis is out of the plane defined by the p-polarization of the Brewster windows. The term p-polarization refers to polarization where the electric vector of the light lies in the plane of the incident ray and the normal to the surface and is the normal mode of operation of such lasers. The polarization of light is the direction of its electric vector as opposed to the orientation of the magnetic vector. It is necessary to let the plate be tilted at Brewster's angle, because at that angle there is no reflection from an incident p-polarized beam.
Birefringence is the property of some materials which are homogeneous but anisotropic to have a different index of refraction for light traveling through the material in different directions. That is, the velocity of a light wave in such material is not the same in all directions. In such materials, two sets of Huygens wavelets propagate from every wave surface, one set being spherical and other being ellipsoidal. A consequence of this property of anisotropic crystals is that a ray of light striking such a crystal at normal incidence is broken up into two rays as it enters the crystal. The ray that corresponds to wave surfaces tangent to the spherical wavelets is undeviated and is called the "ordinary" ray. The ray corresponding to the wave surfaces tangent to the ellipsoids is deviated, even though the incident ray is normal to the surface, and is called the "extraordinary" ray. If the crystal is rotated about the incident ray as an axis, the ordinary ray remains fixed, but the extraordinary ray revolves around it. Furthermore, for angles of incidence other than zero degrees, Snell's law holds for the ordinary but not for the extraordinary ray, since the velocity of the latter is different in different directions.
The index of refraction for the extraordinary ray is therefore a function of direction. There is always one direction in such a crystal for which there is no distinction between the ordinary and extraordinary rays. This direction is called the optic axis.
Brewster's angle, sometimes also called the polarizing angle, is the angle of incidence for which the reflected ray and the refracted ray are perpendicular to each other. When an incident light beam strikes a boundary between two materials having two different indices of refraction, part of the incident light energy is reflected in a reflected beam and part of it is refracted through the second material. The perpendicular relationship between the reflected ray and the refracted ray is only true when the angle of incidence of the incoming ray is equal to Brewster's angle. This is an angle between the direction vector of the incoming light beam and the normal to the surface defined by the junction between the materials of two different indices. When the incident ray is at Brewster's angle, the angle of refraction becomes the complement of the angle of incidence, so that the sine of the angle of refraction is equal to the cosine of the angle of incidence.
Wavelength selection and tuning through the use of a birefringent plate in the resonator cavity comes about because the plate defines two different axes of retardation for laser energy whose electric vector is polarized along these axes. That is, retardation is the phase difference which builds up between the ordinary and extraordinary ray as they travel through the birefringent material at different speeds. When the retardation corresponds to an integral number of full wavelengths, the laser operates as if the plates were not there, i.e., in the p-polarization of the Brewster surfaces. At any other wavelength, however, the retardation is not an integer number of wavelengths and the laser mode polarization is shifted by the plate and suffers losses by reflection of energy out of the cavity with each encounter with a surface at Brewster's angle.
Tunability is achieved by rotating the birefringent plate in its own plane, because this changes the included angle between the optic and laser axes (called the tuning angle) and, hence, the effective principal refractive indices of the plate are angularly rotated. The losses imposed by the birefringent plate on wavelengths whose retardation is other than an integer multiple of one wavelength therefore prevent lasing action. This is because lasing action only occurs when the output energy exceeds the input energy, and this condition will be not be true at wavelengths for which the losses are too great. Therefore, lasing only occurs at a particular wavelength or group of wavelengths within the selected range having phase retardation which is at or near an integer multiple of one wavelength.
In a paper by Holtom and Teschke, "Design of a Birefringent Filter for High-Power Dye Lasers", IEEE Journal of Quantum Electronics, Vol. QE-10, No. 8, pp. 577-579 (August 1974), the design of birefringent filters for the suppression of sidebands in dye lasers was discussed. The paper explains that in using a birefringent filter for wavelength tuning of a continuous-wave dye laser, several crystalline quartz (quartz is birefringent) plates are inserted within the laser cavity at Brewster's angle. These laser plates both retard and polarize light passing through the cavity. It is explained that this is a variation of the Lyot filter which has separate retarders and polarizers. The tuning of such a laser structure is accomplished by rotating the assembly of quartz plates about an axis normal to the surface of the quartz plates.
The advantages of such a filter are explained to be low loss, high dispersion, small physical size, resistance to damage at high intensity, and the absence of any reflecting surfaces normal to the laser beam. However, for such a filter to be suitable for a high-gain dye laser, the lasing and transmission of light at frequencies other than at the desired passband must be reduced to approximately 10-20%. That is, in high-gain lasers, even though there are losses imposed on light outside the desired passband, lasing action can still occur unless the loss imposed on light outside the passband is sufficiently large to prevent the lasing criteria from being met. The problem addressed by this reference is that of incomplete polarizing leading to undesirable sideband operation. That is, incomplete polarizing action of the Brewster surfaces of the quartz plates leads to sideband transmissions. Attenuation of these sideband peaks may be increased by adding glass plates to the stack of quartz disks in order to increase the polarizing efficiency. This increased polarizing efficiency causes greater losses for light having the frequencies in the sidebands, thereby suppressing these undesired modes of operation.
In a paper by November and Stoffer, "Derivation of the Universal Wavelength Tuning Formula for a Lyot Birefringent Filter", Applied Optics, Vol. 23, No. 4, pp. 2333-2341, the authors discuss the nature of Lyot filters and their function in tuning to provide variable monochromatic transmission of light at wavelengths spanning the useful operating range of the filter. The authors explain that a Lyot birefringent filter is functionally a series of tuning elements each of which rotates synchronously with the other tuning elements so that the whole collection of tuning elements act as a fixed unit. Each tuning element has three optical components: an entrance polarizer, a birefringent crystal, and a quarter-wave plate. The entrance polarizer is fixed with respect to the birefringent crystal so that it divides the light in equal intensity between the two axes of different refractive index in the birefringent crystal while preserving one temporal phase; the phase of the light wave is advanced in the crystal's extraordinary axis over its ordinary axis in the birefringent crystal. The quarter-wave plate following the birefringent crystal changes the phase-lag or differential retardation modulo pi into a specific angle of linear polarization. Those wavelengths of light that experience a specific fractional differential retardation are selected by a following polarizer; usually this is the entrance polarizer of the next tuning element in the series.
The partial tuning formula given by November and Stoffer at page 2334 has been found experimentally by the applicant to be only partially correct. This paper does, however, give a nice background discussion of how the birefringent filter elements work. It is there stated that the linear polarizer that follows a tuning element shows maximum transmission at those wavelengths of light that experience a specific phase-delay modulo pi in passing through the birefringent crystal. The phase delay is the retardation of the crystal. The retardation is stated by the authors to be a function of the difference in refractive indices, i.e., the birefringence of the crystal, the thickness of the crystal, and the wavelength of the light. The transmission factor is stated to be a function of the phase delay and the angle between the tuning element and the following polarizer. As the tuning element rotates relative to the following polarizer, the maxima of transmission shifts in wavelength by the fraction of a fringe equal to the angle divided by pi. Each one-half rotation of the tuning element shifts the wavelength maxima of transmission through one full fringe and is said to span the spectral range of the tuning element.
In the Lyot design, the successive tuning elements are twice the respective thickness of the previous element at its nominal operating wavelength. All the tuning elements are rotated against following polarizers to provide maximum transmission through the tuning elements at a single wavelength in a process called "alignment". This single wavelength is called a tune solution and gives a profile shown in FIG. 1 of the paper authored by November and Stoffer. The multiple passbands are spaced by the free spectral range of the thinnest tuning element.
In another paper by Preuss and Gole, "Three-Stage Birefringent Filter Tuning Smoothly over the Visible Region: Theoretical Treatment and Experimental Design", APPLIED OPTICS, Vol. 19, No. 5, pp. 702-710 (March 1980), it is stated that the technique of frequency selection in a dye-jet laser takes advantage of the fact that a low-gain laser can operate only with a polarization that is transverse magnetic (TM) with respect to any intracavity elements oriented at Brewster's angle (windows, dye jets, or the birefringent plates, themselves). A single birefringent plate has the property of transforming the incident TM polarization into some elliptical polarization composed of both TM and TE (transverse electric) linear polarization components. Conventionally, a light wave has orthogonal electric and magnetic vectors, and its polarization is stated to be the direction of the electric vector. The authors go on to state that the power transformed from the TM mode into the TE mode is no longer available to stimulate emission in the lasing TM mode. If the power loss is sufficient, lasing will cease.
The power loss due to a birefringent filter is a function of the orientation of the birefringent plates as well as the frequency of the radiation passing through these plates. The thicknesses and orientations of the component plates in a birefringent filter are chosen so that there will be one frequency within the gain curve of the laser medium for which the polarization will be unaffected. Since this frequency component alone suffers the minimum loss, it will continue to lase while all other frequencies are suppressed. By changing the orientation of the filter, the frequency of minimum loss is changed and the laser is thereby tuned.
A useful filter must be efficient. As such, a filter tuned to the peak of the laser gain curve should cause little or no degradation of the output power. It is also important that the birefringent filter display smooth continuous tuning as the orientation of the device is gradually altered. The authors state that this latter quality appears to be lacking for certain wavelength regions in commercially available filters. The authors go on to state a relationship between the wavelength and various angles in a typical system. However, it has been found by the assignee hereof that this relationship does not fit experimental data, and is therefore suspect.
In another paper by Lovold, et al., "Frequency Tuning Characteristics of a Q-Switched Co:MgF.sub.2 Laser", IEEE Journal of Quantum Electronics, Vol. QE-21, No. 13, March 1985, pp. 202-207, the authors present the most complete relationships of any known in the literature, covering the most general case where the optic axis can be at any arbitrary angle with respect to the plate normal. Because of the complexity of the relationships, a simple closed form equation cannot be derived and is not taught; moreover, the parameters are not all directly measurable. If the angle sigma is set to 90 degrees, then the relation can be shown to be the same as the relation shown in Appendix A (described more fully below), by making appropriate substitutions.
The authors claim that it is desirable for their laser to use birefringent plates with the optic axis at an angle of 35 degrees to the normal to achieve a high modulation depth; however, it is apparent that the tuning is a very steep function of the rotation angle, rho. In the dye laser cases considered in this application, steep tuning functions are undesirable. Another reason why it is undesirable to choose solutions described in this paper is that the design is very sensitive to errors in plate thickness; therefore, for both these reasons, solutions presented herein are restricted to the case where sigma equals 90 degrees and the optic axis is near -40 degrees to the plane of incidence.
There are several problems with the prior art continuous wave tunable dye lasers with respect to the prior art tuning structures. The first problem is that there is no accurate tuning equation which is available to predict the selected wavelength of lasing for a given angle of the tuning mechanism. At least one purported tuning equation has been put forth by the prior art as noted in the above discussion of the references. However, this tuning equation has been found by the applicant to not fit experimental data. The equations of the Lovold et al. reference are useful, but not specifically applicable to the cases of interest for a visible light dye laser.
The thickness of the thinnest element of the birefringent filter tuning structure determines the characteristic tuning curves of the tuning structure. One common thickness in the prior art for the thinnest element is approximately 0.33 mm. Another thickness that has been used in the prior art is 0.381 mm. Neither of these thicknesses alone provides a single mode curve for all dyes; for example, the 0.33 mm thickness requires two orders to cover the 800-900 nm range, but is satisfactory (has one order curve) in the 700-800 nm range. The opposite is true with the 0.381 mm thickness: that plate can tune over 800-900 nm on a single order curve, but not over 700-800 nm. The difficulty that this creates is that the operator must operate on one mode curve or another, and since no mode curve encompasses the entire range, such operators must operate on two mode curves to tune the laser throughout the entire range. This is inconvenient since it requires the operator to tune out to the extreme ends of one mode curve and then to turn the birefringent filter a complete revolution to get to the next mode curve before tuning can resume. This is both inconvenient and causes errors when operating near the end of either mode curve. As an example of these types of problems, sideband rejection is much worse at the large tuning angles which must be used to reach the ends of any particular tuning curve to reach another mode curve. If sideband rejection is insufficiently high, the laser may lase at unwanted frequencies. Therefore, a need has arisen for a structure for tuning a continuous wave dye laser smoothly through each of the various dye ranges on a single order curve.
There has therefore arisen a need for a method of predicting the tuning characteristics of particular structures based upon a tuning formula such that the appropriate thickness for birefringent tuning elements may be chosen to achieve single-mode curve tuning throughout the desired range with good linearity and good sensitivity. Without an accurate tuning formula, the designer of a laser is left to choose between an infinite number of thickness for the birefringent tuning plates. This requires a great deal of experimentation to find the proper thickness to achieve smooth, single-mode curve tuning throughout the desired range.
Another problem which has arisen in the prior art of continuous wave dye laser tuning is that of wavelength jump. It has been noted that at high pumping energies, when a continuous wave dye laser is tuned near the end of its tuning range, there occurs a shift in wavelength back toward wavelengths in the center of the tuning range. This is an undesirable feature, since the desired tuning characteristics of a CW dye laser are to smoothly tune throughout the range of operation with no discontinuities in the wavelength versus tuning angle relationship. When the ends of the tuning range are reached, it is desired that the laser simply go out, i.e., stop lasing. Therefore, there has arisen a need for a tuning structure for a CW dye laser which can eliminate these wavelength discontinuities near the end of the tuning range.